Optical flaw detection apparatus



Dec. 3p, 1958 Filed Sept. 29. 1954 I E. D.' cooK OPTICAL FLAw DETECTIONAPPARATUS 2 Sheets-Sheet l PULSE sHAP/NG c/Rcu/T ELECTRON CAMERANEGATIVE ems PoTENTmL b SUPPLIED To THYRATRQN 9 TINE Inventor' EllsworthD. Cook,

H/'S Attorney.

Dec. 30, 1958 E.V D. cooK 2,865,376

OPTICAL FLAW DETECTION PPARATUS Filed sept. 29. 1954 2 sheets-sheet 2Inventor- E /lswofth D. COOK nited States arent O OPTICAL FLAW DErEcTioNAPPARATUS Elisworth D. Cook, Scotia, N. Y., assigner to General ElectricCompany, a corporation of New York Application September 29, 1954,SerialNo. 459,073

6 Claims. (Cl. 88-14) The present invention relates to Va new andimproved indicating circuit for flaw inspection apparatus.

More specifically, the invention relates to an indicating circuit forfiaw inspection apparatus which is' capable of deriving output electricsignals indicathe of different types of flaws occurring in a materialunder inspection.

There are a number of known flaw inspection apparatus which can be usedfor inspecting a moving strip of material such as paper, tin, sheetsteel, etc. for defects occurring therein, and to derive automaticallyan output signal indicative of the occurrence of a defect in thematerial. One such inspection apparatus is disclosed in patentapplication Serial No. 145,458, tiled February 2l, 1950, E. S. Sampson,Blemish and Opacity Indicator, now abandoned, and assigned to the sameassignee as the present invention. The number of uses for inspectionapparatus of the type disclosed in the above-identitied application havebeen somewhat restricted, however, due tothe fact that known apparatusof this type are incapable of providing suicient information concerningthe ilaws detected.

It is, therefore, one object of the present invention to provide a newand improved output circuit for sheet material inspection apparatuswhich is capable of dif'- erentiating between aws occurring in thematerial in the direction of travel of the material, such as a crease ora fold, and flaws which are concentrated at substantially one point onthe material.

Another object of the invention is to provide an output circuit for flawdetection apparatus 'which is capable of distinguishing between flaws ofdifferent character, and which is entirely reliable in operation.

A further object of the invention is to provide an output 'circuit foriiaw detection apparatus Ihaving the aboveset forthrcharacteristics,which is simple in convided for material inspection apparatus whichincludes an electron optics device. Although it is not limited to `suchmeans as for example inspection and scanning can be provided by spacedand insulated electrodes contacting a conductive medium' and operatingin conjunction with a `commutation device to connect each electrodeseparately tothe output circuit. l In the present example, however,koptical means are used, and are disposed intermediate the electronoptics device and the material to focus an optical image of the materialadjacent the in spec'tion equipment on the electron optics device.Circuit means are coupled to the voutput lof the electron'optics devicefor ldiiferentiating between different types of flaws occurring in thematerial under inspection, and in the preferred embodiment of theinvention, this circuit means 'comprises a storage circuit coupled tothe output of the electron optics device for deriving an indication ofthe relatively large and extensive aws occurring in the material underinspection. Also a ilaw pulse countcontrolled circuit is provided, andis coupled to the out- A'put of Vthe electron-optics device in parallelwith the fice storage circuit for deriving an indication ofcomparatively small aws occurring in the material under inspection.

Other objects, features, and many of the attendant advantages of theinvention will be appreciated more readily as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein likeparts are identied by the same reference character, and wherein:

Fig. l is a schematic circuit diagram of ilaw detection apparatus whichincludes a novel output circuit constructed in accordance with theinvention as a part thereof;

Fig. 2 is a graph showing the voltage vs. time charging characteristicsof a capacitor comprising a part of the output circuit of the apparatusshown in Fig. l; and

Figs. 3ft-3f are a series of graphs illustrating the chargingvoltage-time characteristics of a portion of the output circuit of theapparatus shown in Fig. l.

The flaw detection apparatus shown schematically in Fig. l of thedrawings is designed to detect flaws in a moving strip of material 11.For this purpose, the strip of material l1, which may be a moving sheetof paper, is 'lluminated from `a light source 12, the light rays thereofbeing colurnnated by a columnating lens L13, and projected upon thesurface of the moving sheet of paper. The illuminated area of the movingstrip of material 11 is imaged upon an electron camera device 14 by apro jection lens assembly i5 wherein an electric signal rep resentativeof `the condition of the surface of the strip of material is produced.The electron optics device 14 preferably comprises an electron camerasuch as an image dissector, an orthocon, an iconoscope, or a phototubeand associated mechanical scanner arrangement, an image dissector beingpreferred because of the signal to noise ratio at the high scanningrates at which it is capable of operating. Sweep signals are supplied tothe electron optics device 14 from a sweep generator circuit, and theoutput electric signals developed by the electron optics device aresupplied through an amplifier 17 to a pulse shaping circuit 13 which incertain applications might be a differentiating circuit and outputamplier 19. The construction and operation of the ilaw detectionapparatus thus far described is entirely similar to the apparatusdescribed in the above-identified patent application of E. S.

Sampson. Hence, for a more detailed description 'of the operation andconstruction thereof reference is made to such application. Briey,however, the electron-optics device operates in the following manner:sweep generator i6 is connected tothe horizontal scanning element of theelectron optics device 14 to cause the same to scan the optical imageformed at the camera transversely to 'the direction of motion of thestrip of material lli. Preferably, the scanning rate is at least -equalto the quotient of the speed that the strip of material 11 travelsdivided by the width of the scanning line so that the entire suace ofthe strip 11 is viewed by the camera. For example, if the scanning spotot the camera covers an area of the optical image of strip 11 equivalentto a circle 2O mils in diameter on the strip 11, and the strip travels aspeed of 1500 feet per minute, the horizontal scanning rate should be atleast 15,000 lines per second. As long as the surface of the sheet ofmaterial 11 remains at some predetermined standard quality, no outputelectric signal is produced by the electron camera 14, however, upon theoccurrence of a blemish, fold, or `crease in the surface of strip 11, anoutput signal pulse is produced by the electron optics device 14 whichis passed through the amplifier 17, rectified by detector 18, andsupplied vto a suitable output circuit through output amplifier 19 -toprovide an Output indication of the occurrence of the flaw.

While the fiaw detection apparatus heretofore known have beensatisfactory for many applications, they have not been sufficientlyrefined to differentiate between different types of flaws, some of whichmight be tolerated over others. For example, consider a crease or foldthat extends in the direction of travel of the moving strip of material,such as is illustrated at 21. From a consideration of this crease it canbe seen that as a scanning of electron optics device 14 is carried out,a pulsed' electric output signal is produced each time the scanning beamof the electron optics device crosses through crease 21. Consequently,over a period of time, a series of periodic or nearly periodic signalpulses depending upon or fold has a rather large dimension that extendsin the v direction of movement of strip of material. Consider now a liawsuch as a spot shown at 22 on the strip of material, it can beappreciated that only one or possibly v two at the most, electric outputpulses will be produced as a scanning beam of the electron optics device14 passes through the iaw. Consequently, the types of electric outputsignals produced by the electron optics device 14 do differ in characterso that by distinguishing such differing signals, a means can beprovided for differentiating between the different types of iiaws. Inthe event that the quality requirements for a strip of material cantolerate a longitudinally extending fold or crease, such as shown at 21,then such liaws can be distinguished from other types of fiaws whichcould not be tolerated. In this manner, fiaw detection apparatus of thekind shown in Fig. l, can be applied to a large number of additionalapplications for which it heretofore could not be used. The presentinvention provides an output circuit means for flaw detection apparatuswhich is capable of differentiating such varied types of flaws.

The output circuit provided by the present invention for the fiawdetection apparatus includes a storage circuit means indicated generallyat 23 for deriving an indication of relatively large and extensive flaws(that is, iiaws exceeding some predetermined size) occurring in thematerial under inspection which have a considerable dimension extendingin the direction of travel of the moving strip of material being gaged.The storage circuit means comprises a charging capacitor 24 coupled tothe output of amplifier 19 through a diode rectifier 25. The rectifier25 has the cathode thereof coupled to the output of electron opticsdevice 14 through a coupling capacitor and a phase inverting amplifier26 and is clamped to ground by a clamping diode 27. By reason of theconstruction, only positive-going charging pulses are supplied to thecharging capacitor 24 through the diode rectifier 25. Coupled across thecharging capacitor 24 is a grid resistor 28 which is in turn connectedto the control grid of a grid controlled gas discharge tube 29. Gasdischarge tube 29 has a grid biasing potential supplied thereto from abattery source of electric energy 30 through grid resistor, and has asuitable indicator such as a relay 32 connected to the plate circuitthereof. The biasing resistor 2S serves to apply the negative biassupplied by battery 30 to the control grid of gas discharge tube 29 tomaintain that tube in a normally cut off condition, and also serves as aleakage resistance forthe charging capacitor 24. Needless to say, thetime constant of the RC circuit formed by the charging capacitor 24 andthe resistor 28 is selected so that it is long with respect to theperiod of thescanning cycle (one scanning line) of electron opticsdevice 14.

In operation, flaw signal indicating pulses produced by the electronoptics device 14 are supplied from the pulse shaping circuits andamplifiers 18 and 19, respectively, and through rectifier 25 to thecharging capacitor 24. In the event that the particular flaw beinglooked at by the optical system of the detection apparatus is elongatedin the direction of movement of the strip 11 such as that shown at 21, arepetitive series of such pulses will occur. By applying this series ofpulses to the charging capacitor 24 through rectifier 25, the totalcharge on the charging capacitor will build up in the manner indicatedin Fig. 2 of the drawings wherein it is seen that as each subsequentpulse is applied to the charging capacitor, the total charge across thecapacitor increases a predeter mined amount. To prevent any accumulationof charge on capacitor 24 by negative-going pulses received fromamplifier 19 with consequent blocking of the circuit, the plate of therectifier 25 is returned to ground through the clamping diode 27. Hence,it is assured that the charge on charging capacitor 24 will build up ina linear manner as a result of the periodically recurring aw indicatingpulses applied thereto, and ultimately reaches a value sufficient toovercome the negative bias supplied lo the control grid of gas dischargetube 29. Upon this occurrence, the gas within the tube ionizes, and thetube is rendered conductive allowing current to flow through the relaywinding of relay 32 and providing an outward indication of theoccurrence ofa liaw. From a consideration of the charging voltagecharacteristic curve shown in Fig. 2, it can be appreciated that onlyflaws which have a long dimension extending in the direction of travelof the strip of material 11 produce a series of output pulses in theelectron optics devices 14which will be capable of building up asufficient charge on the capacitor 24 to overcome thehias on the controlgrid of the gas discharge tube 29. Hence, the storage circuit 23produces an indication of fiaws of this nature only.

In addition to the above-described circuitry. alfiaw pulse countcontrolled circuit means indicated generally at 35 also is connected tothe output of the electron optics device 14 in parallel with the storagecircuit means 23. The flaw pulse count controlled circuit means isprovided in order to detect flaws of less than the predetermined sizeand comprises a gating circuit shown at 36 adapted to control theoperation of a second grid controlled gas discharge tube 37 inconjunction with a trigger circuit 38 coupled across the output of theelectron optics device 14 in the manner shown. l

The trigger circuit 38 comprises a pair of cascade connected electrondischarge tubes 39 and 41 having the cathodes thereof connected tocathode load resistors 42 and 43, respectively, and the anodes thereofconnectedthrough suitable plate load resistors to a source of positiveplate potential. The anode of the electron tube 39 is connected througha suitable resistor coupling circuit to the control grid of the electrontube 41, and the control grid thereof is connected to a short timeconstant charging network comprising a resistor 44 and a capacitor 45.The short time constant charging network thus comprised is connectedacross the output of electron. optics device 14 through a unidirectionalcoupling device com prising a rectifier 46 having the negative electrodeelement thereof connected directly to the output of the amplifier 19 sothat only negative-going flaw signal pulses are passed to the short timeconstant charging network 44, 45. The network 44, 45 is designed to havea time constant in the order of 2t where t is the approximate period ofone scanning cyclev (one scanning line) of electron optics device `14.Consequently, the charging network serves to apply a negative keyingsignal to the control grid of the first electron discharge tube 39 inthe trigger circuit, and as tube 39 is designed to normally beconductive, the application thereto ofthe negative positive potential isremoved from the cathode of the vacuum tube 52 therefore thebiasedvoltage of vacuum tube 37 drops to the potential of the battery 59. Theoutput of trigger Circuit 38 also is supplied through a conductor 48connected to the input of the gating circuit 36.

Gating circuit 36 comprises a rst electron discharge device 51vhavingthe anode thereof coupled through a resistor coupling network to thecontrol grid of a second electron discharge device 52. The anodeeiectrode of electron discharge device `52 is turn coupled back througha capacitor S3 to the control grid of electron discharge device 51, andtogether with a grid biasing resistor 54 likewise connected to thecontrol grid of electron discharge device 51, forms aresistance-capacitance charging network whose time constant is chosen inaccordance with the size of the flaws to be detected. The control gridof electron discharge device 51 is also connected to the output oftrigger circuit 38 through a conductor 48 and a unidirectional couplingdevice 55, and the signal supplied to discharge device 51` through thisconnection serves to control the operation of the gating circuit. Theoutput of the gating circuit 36 is supplied through a conductor S6connected to a load resistor 51a` in the cathode circuit of electrondischarge device 51 and an isolating capacitor S7 Vto the control gridof the second gas discharge tube 37.

The gating circuit 36 comprises a conventional one-- shot multivibratorwhose construction and operation is well-known in the electron circuitryart. Briefly, however, the circuit is biased so that electron dischargedevice 51 normally is conducting and device 52 is nonconducting. Uponthe application of a negative aw signal pulseto the control grid ofdevice 51 from the trigger circuit 38, the device 51 is renderednon-conductive, and thepositive voltage which was produced across thecathode load resistor 51a is removed. Upon the removal of this Voltagethe voltage diierentiating action of the capacitor 57, resistor 61combination caues a negative-going pulse to be applied to they grid ofgas discharge device 37. Simultaneously, a negative charge is built upon capacitor S3 which maintains discharge device 51v cutoff until suchtime that the charge leaks ol`- through resistor 54 whereupon dischargedevice 51 again becomes conductive. The time constant of theresistor-capacitor network. 53, 54 determines the amount of timerequired for the charge on capacitor 53 to leak off, and hence theperiod of an operating cycle of the circuit. At the end of this time,discharge devicel 51 starts to conduct again and a positive voltage isagain produced at the upper end of the cathode load resistor 51a ofdischarge device Si, which` is supplied to the control grid of gasdischarge device 37 as a positive-going pulse.

The second gas discharge tube 37V has a negative bias supplied theretofrom ya battery 59 connected to the control grid thereof through abiasing resistor 61, and has an indicatorV such as a relay 62 connectedin the plate circuit thereof. Energizing potential is supplied to tube37 from a source of alternating potential (not shown) connected to theplate of gas discharge tube 37 through the eld winding of the indicatorrelay 62.

The operation of the Flaw pulse count control circuit means can best beunderstood by reference to Fig. 3 of the drawings, wherein a graphrepresentative of a series of negative-going flaw signal pulses is shownin Fig. 3a of the drawings. These negative llaw signal pulses areapplied through the rectifier 46 to the short time constant chargingnetwork 44, 45. Assuming that the pulses have been produced by a flawsuch as 21 occurring in the strip of' material 11 under inspection, andthat the periodic scanning ofv the electron optics device has resultedin the production of the series of pulses, the pulses may be saidv tohave a period t representative of one scanning cycle.. The short timeconstant charging network 44, 45 is designed tohave a time constantapproximately equal to .2t4 so that-it serves to supply a negativecut-orf signal to the control grid of the electron discharge device 39tor a period of time equal to the number of pulses occurring in anyseries plus an additional period 2t required for the network todischarge. The negative biasing signal supplied to the control grid ofelectron discharge device 39 serves to cut ot this device which normallyis conductive.Y Upon electron tune 35 being rendered non-conductive, thepositive voltage from the upper end of the cathode resistor 42 whichrisapplied to the ycontrol grid of gas discharge device 37 through theconductor 4/ is removed. Since the gas discharge tube 37 already isbiased non-conductive by the grid biasing source 59, removal of thispositive grid voltage has no eiect on the device. The electron dischargedevice 41 is rendered conductive. However, due to the increase in platepotential of the electron discharge device 39, a negative-going signalpulse is produced in the plate circuit or"- device 41 that is suppliedto the control gridvof electron discharge device 51 in the gatingcircuit. The electron discharge device 51 normally is conductive so thatthe application of a negative pulse thereto from a trigger circuitrenders this device non-conductive. Electron Adischarge devices 51 and52 comprise a one shot multivibrator which will return to its initialstate of operation after a period determined by the time constant of theresistor-capacitor network formed by capacitor 5,3 and resistor 54.Consequently, the gating circuit 36'is keyed on and orf by thenegative-going triggering pulses supplied thereto by a trigger circuit38, and is caused to pais through one cycle of operation for each seriesof negative going Haw signal pulses supplied to the trigger circuit.Upon the gating circuit 36 being triggered from its initial operatingcondition to the other thereof, the electron discharge device 5.1becomes non-conductive thus resulting in a removal of the positivedirect current potential at the upper end of the cathode load resistor51a. Upon removal of the positive unidirectional potential from thecathode load resistor 51a a negative voltage pulse is supplied to thecontrol grid of the gas discharge device 37 due to the differentiatingaction of the capacitor S7. As gas discharge device 37 is notconducting, the negative signal pulse supplied thereto in this mannerhas no etfect on its operation. However, up'on the gating circuit 36returning to its initial condition due to the charge on capacitor 53having leaked olf through resistor 54 and discharge device 51 havingagain become conductive, a positive potential is produced across thecathode load resistor 51a of electron discharge device 51 which issupplied to the control grid of gas discharge device 37 as a positivepulse. Should this positive going pulse coincide with or follow afterthe occurrence of a positive potential supplied to the control grid ofdischarge device 37 from the load resistor 42 of electron dischargedevice 39 in trigger circuit 38, the gas discharge device 37 is renderedconductive, and an output indication of a aw is provided by thiscircuit.

The sequence of actions described above is best depicted in Figs. 3athrough 3f of the drawing. For the purpose of illustration, assume thatit is desired to detect flaws of a size which will produce four or lessaw signal pulses. It should be understood, however, that the circuit canbe adjusted to operate on any desired number of aw signal pulses, andhence any desired size flaw. The gating circuit 36 is designed to have aperiod of operation Trl-2t where T1 is a total time elapsed or requiredfor the chosen maximum number of flaw signals to be produced by adesired flaw size. lf it is necessary to detect flaws of greater size orof less size, T1 can be varied in length to accommodate this need. Uponthe occurrence of a tlaw of such'a size to produce four flaw signalpulses passing within the view of electron optics device 14, then, asshown in Fig. 3b (which depicts the grid voltage of electron dischargedevice 39), the electron discharge dievise 39 in tlie'trigger circuit 38is rendered non-conductive for a period of time T1 plus the time 2rrequired for the 7' *charge on short time constant charging network 44,45 to leak off.A Fig. 3c depicts the change in voltage appearing acrossthe resistor 42 with time upon such occurrence wherein it can be seenthat the potential across resistor 42 drops'to zero and does not riseagain to its normal positive value until a period Trl-2r later.

The signal pulses produced across the load resistor 51a in the cathodecircuit of electron discharge device 51 are of the same generalconfiguration as that illustrated in Fig. 3c and will be of the sameduration (Tft-2t) each time the discharge device 51 is cut oli. Thecurrent vow in the circuit, which includes load resistor 51a, isolatingcapacitor 57, and biasing resistor 61 which results from the voltageproduced across the load resistor 51a, is illustrated in Fig. 3d of thedrawing. It is seen that `upon electron discharge device 51 beingrendered nonconductive, a negative current pulse flows through thecircuit and at the end of a period Trl-2r a positive current pulseiiows. The resultant voltagedrop across the biasing resistor 61 thenwill have substantially the same wave shape. By combining the graphsshown in Figs. 3c and 3d, a composited graph 3e is shown which depictsthe voltage values appearing across the resistor 61 due to the rnegativepulses from both load resistors 42 and 51a. From Fig. 3e of thedrawings, it can be appreciated that the negative bias supplied to thecontrol grid of the gas discharge device 37 by the biasing battery 59 isadjusted so that the normal direct current ow through resistor 61'by theD. C. potential supplied from resistor 42 is below the reference orcut-off value of this gas discharge tube, in this instance assumed to bezero volts. Upon the electron discharge device 39 being lrenderednonconductive due to the `occurrence of a negative flaw signal pulse,this potential (from resistor 42) is reduced to 'Zero s0 that the fullvalue of the bias potential supplied by the battery 59 is letlective tomaintain gas 'discharge tube 37 at cut-oit. Upon the end of the periodTrl-2f, however, the bias potential supplied from resistor 42 throughconductor 47 is again applied to the control grid of the gas dischargedevice. This potential, when combined with the positive pulse suppliedby the gating circuit 36 and produced as a result of the electrondischarge'device 51 again being rendered conductive at the end of onecycle of operation, raises the potential of the control grid of the gasdischarge device 37 above the cut-off value, and causes the device tobecome conductive. The resultant plate current flow is indicated in Fig.3f of the drawing, wherein it is seen that a positive going pulse isproduced which is suticient to actuate the indicator relay 62. The shortpulse which appears on this current Wave form is due to the excitationvoltage applied to the plate of the discharge device 37. In the eventthat more than the predetermined number of iiaw signal pulses occur, itcan be appreciated that the no direct current biasing potential suppliedthrough conductor 47 from the cathode load resistor 42 to the controlgrid of gas discharge device at a time suicient to coincide with theproduction of the positive going pulse supplied by the gating circuit36. Consequently, the circuit will not respond to flaws of a size largeenough to produce more than four iiaw signal pulses in any one series,in the output of the electron optics device 14. However, should there befour aw signal pulses, or fewer, it can be appreciated that the directcurrent biasing potential supplied from the cathode load resistor 42through conductor 47 to the control grid of gas discharge device 37,will be present upon the production of the positive going pulse pro-Iduced by the gating circuit 36 at the end of one cycle of operati-onthereof, and hence an indication will be produced in the output circuitthat a desired size flaw has occurred in the material under inspection.Consequently, it is believed clear that the flaw signal count-controlledcircuit means described above is capable of selecting ilaws having anydesired size, and indicating the occur- Vrence of such aws.

` Prom' the foregoing description, it can be appreciated that theinvention comprises a new and improved output circuit for aw detectionapparatus which is capable of differentiating between aws occurring inthe material which have some length in the direction of travel of thematerial such as a crease or a fold, and aws which are concentrated atsubstantially one point and are relatively small. Additionally, thecircuit for accomplishing this is entirely reliable in operation, and isrelatively simple in design and inexpensive to manufacture.

Obviously, other modifications and variations of the present inventioncan be suggested by those skilled in Vthe art in the light of the aboveteachings. It is, therefore, to be" understood that changes may be madeherein which are within the full intended scope of the present inventionand as defined by the appended claims.

Whatl claim as new an-d desire to secure by Letters Patent of the UnitedStates is:

l. ln material inspection apparatus which includes optical meansdisposed to focus an optical image of the portion of the materialadjacent an inspection apparatus on an electron-optics device and meansfor providing scanning of the image by the electron-optics device, theimprovement of a storage circuit comprising a unidirectional conductingdevice o-peratively coupled in series circuit relationship with anenergy storage device across the output of said electron-optics device,a grid controlled discharge tube for providing an outputsignal havingsaid energy storage device coupled thereto, said apparatus furtherincluding a gating circuit coupled across the output of saidelectron-optics device, a second grid-controlled gas discharge tube forproviding a second output signal having the control grid thereofoperatively coupled to the output of said gating circuit, and a triggercircuit voperatively coupled to the output of said electron-opticsdevice and having the output thereof coupled to said gating circuit andto the control grid of said second discharge tube in parallel withgating circuit for controlling the action of the same.

2. ln material inspection apparatus which includes optical meansdisposed to focus an optical image of the portion ot' the materialadjacent the inspection equipment lon an electron-optics device andmeans for providing scanning of the image by the electron-optics device,the improvement of storage circuit means coupled to the output of saidelectron-optics device and co-mprising a grid controlled dischargedevice having a charging capacitor connected in the control grid circuitthereof, said storage circuit means serving to derive an indication offlaws occurring in the material under inspection which have a relativelylarge dimensio-n perpendicular to the direction of scanning, and flawpulse count controlled circuit means coupled to the output of saidelectronoptics device and comprising a second grid-controlled dischargedevice, a gating circuit operatively coupled to said discharge devicefor supplying a gating signal thereto, a trigger circuit coupled to thecontrol grid of said second discharge device in parallel circuitrelationship with said gating circuit, and coupled to control saidgating circuit, and a coupling circuit for coupling said trigger circuitto the output of said electron-optics device, said aw pulsecount-controlled circuit means serving to derive an indication of awsoccurring in the material under inspection having a comparatively smalldimension perpendicular to the direction of scanning.

3. ln material inspection apparatus which includes a scanningelectron-optics device and optical means disposed intermediate saidelectron-optics device and said material for focusing an optical imageof the portion of the material adjacent the inspection apparatus on theelectron-optics device, the improvement of a storage circuit comprisinga diode rectifier operatively coupled in series circuit relationshipwith a charging capacitor, said series circuit being connected toreceive the output of said electron-optics device, a grid-controlled gasdischarge tube having said charging capacitor connected in thecathode-control grid circuit thereof, and an indicating device connectedin the plate circuit of said gas discharge tube, said apparatus furtherincluding aw pulse countcontrolled circuit means coupled across theoutput of said electron-optics device and comprising a secondgridcontrolled gas discharge device, a gating circuit having the outputthereof operatively coupled to the control grid of said second gasdischarge device, a trigger circuit having one output thereof coupled tothe control grid of said second gas discharge device in parallel circuitrelationship with said gating circuit and having another output thereofcoupled to the input of said gating circuit, and a short time constantresistor-capacitor charging network coupled across the output of saidelectron-optics device and having the input terminals of said triggercircuit coupled thereacross.

4. In material inspection apparatus which includes optical meansdisposed to focus an optical image of the portion of the materialadjacent the inspection apparatus on an electron-optics device and meansfor providing scanning of the image by the electron-optics device, theimprovement of a storage circuit comprising a diode rectifieroperatively coupled in series circuit relationship with a chargingcapacitor across the output of said electron-optics device, agrid-controlled gas discharge tube having said charging capacitorconnected in the cathodecontrol grid circuit thereof, and an indicatingdevice connected in the plate circuit of said gas discharge tube, saidapparatus further including aw pulse count-controlled circuit meanscoupled across the output of said electron-optics device and comprisinga second gridcontrolled gas discharge tube having an indicating relayconnected in the anode circuit and energized by a source of alternatingcurrent electric energy, a gating circuit comprising a one shotmultivibrator that automatically returns to its initial operatingcondition upon being triggered from its initial operating condition tothe other, a coupling capacitor for coupling the output of said one shotmultivibrator to the control grid of said second gas discharge device, atrigger circuit comprising rst and second grid controlled electrondischarge tubes connected in series, said first electron discharge tubehaving a cathode load resistor operatively coupled to the control gridof said second gas discharge device and said second electron dischargetube having the anode thereof coupled through a unidirectionalconducting device to the input of said one shot multivibrator, and ashort time constant resistor capacitor charging network coupled acrossthe output of said electron-optics device and having the input terminalsof said trigger circuit coupled thereacross,

5. An output circuit for inspection apparatus including in combination agrid-controlled gas discharge device, a gating circuit having the outputthereof operatively coupled to the control grid of said gas dischargedevice, a trigger circuit having one output thereof coupled to thecontrol grid of said gas discharge device in parallel circuitrelationship with said gating circuit and having another output thereofcoupled to the input of said gating circuit and a short time constantresistor-capacitor charging network coupled across the input terminalsof said trigger circuit for supplying input signals thereto, said outputcircuit means serving to provide an indication of only comparativelysmall numbers of aw signal pulses.

6. An output circuit for inspection apparatus including in combination agrid-controlled gas discharge tube having an indicating relay connectedin the anode circuit and energized by a source of alternating currentelectric energy, a gating circuit comprising a one shot multivibratorthat automatically returns to its initial operating condition upon beingtriggered from one operating condition to the other, a couplingcapacitor for coupling the output of said one shot multivibrator to thecontrol grid of said gas discharge device, a trigger circuit comprisingfirst and second grid controlled electron discharge tubes connected inseries, said rst electron discharge tube having a cathode load resistoroperatively coupled to the control grid of said second gas dischargedevice and said second electron discharge tube having the anode thereofcoupled through a unidirectional conducting device to the input of saidone shot multivibrator, and a short time constant resistor capacitorcharging network 4coupled across the input terminals of said triggercircuit, said output circuit means serving to provide an indication ofonly comparatively small numbers of flaw signal pulses.

References Cited in the iile of this patent UNITED STATES PATENTS2,295,000 Morse Sept. 8, 1942 2,393,631 Harrison et al Ian. 29, 19462,494,441 Hillier Ian. 10, 1950 2,563,213 Coleman Aug. 7, 1951 2,594,423Gordon Apr. 29, 1952 FOREIGN PATENTS 714,344 Great Britain Aug. 25, 1954714,350 Great Britain Aug. 25, 1954 157,888 Australia July 28, 1954

